#include <iostream>
using namespace std;
 
double fdif(double x,double y)
{
	return x+y;
}
 
int main() 
{
	double k1, k2, k3, k4, dy, x, y0, a, b, h;
	scanf("%lf %lf %lf %lf", &a, &b, &h, &y0);
	for ( x ; x <= b ; x += h )
	{		
		k1 = fdif (x , y0);
		k2 = fdif (x + h / 2 , y0 + h / 2 * k1);
		k3 = fdif (x + h / 2 , y0 + h / 2 * k2);
		k4 = fdif (x + h , y0 + h * k3);
		y0 += h * ( k1 + 2 * k2 + 2 * k3 + k4 ) / 6;
	}
	printf("%lf", y0);
	return 0;
}

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MC4wIDAuNSAwLjEgMS4w

0.0 0.5 0.1 1.0